This invention relates to intrusion detection systems and more particularly to a fence-type intrusion detection system using an electret cable.
The cable transducer described in U.S. Pat. No. 3,763,482 mounted on a chain link fence is responsive to vibrations in the fence to provide an output signal to processing circuits connected to the transducer. The system described in the foregoing patent has a voltage bandpass amplifier connected to the output of the cable transducer. The amplifier output is rectified and applied to a low pass filter or integrator and when the integrator output exceeds a preset threshold level, an alarm output is generated. Thus, one signal burst is capable of generating an alarm. False alarms from spurious signals can be reduced by increasing the threshold setting but this also reduces system sensitivity.
A signal generated by the electret cable in response to a mechanical stress of the outer conductor produces a minute change in capacitance and thus in electric charge over a small segment of the cable. The small change in charge thus produced must be distributed over the entire cable capacitance when the cable is terminated in a relatively high impedance such as the input to a voltage amplifier. Since cable capacitance is proportional to cable length, the amplifier voltage gain required to increase the signal level to a suitable value increases proportionally to cable length. The maximum length of cable that can be used with each processor is therefore dependent upon the maximum gain available from the amplifier. Furthermore, selection of different lengths of cable at the installation site requires time consuming field adjustments of the voltage amplifier to properly change its gain as required by the cable length selected.
Another problem experienced with the system described in the foregoing patent is difficulty in discriminating against the particular types of false alarm conditions to which the electret cable protected fence may be subjected in a normal environment. Raindrops impacting on the fence can produce signals that result in a false alarm. Another source of false alarm signals is the dragging of a stick along the fence. It is very desirable that the electret cable-equipped fence be capable of better discriminating against such false alarm signals in order that the utility of the system may be broadened.